I have personally tested them in 66, 99, 201 and 401 class machines.
They are of superior quality to the first 66 class bobbins
I stocked which proved unsuitable for machines with the early style Singer
bobbin winder (the gripper hole is too far from the spindle hole) and
customers with 1960s slant needle models have also been thrilled with
them as the little ridge on the cheaper bobbins can interfere with performance.